With the recent spread of portable electronic equipment such as notebook computers, video camcorders, and personal digital assistances, nonaqueous electrolyte secondary batteries having high voltage and high energy density have come to be widely used as a power source. From the concern for the environmental protection, electric-powered vehicles and hybrid-powered vehicles utilizing electric power as a part of motive power have already been put to practical use.
Various additives for nonaqueous electrolyte have been proposed to provide nonaqueous secondary batteries with improved stability or electrical characteristics. Examples of such additives include 1,3-propane sultone (see patent document 1 below), vinyl ethylene carbonate (see patent document 2 below), vinylene carbonate (see patent document 3 below), 1,3-propane sultone or butane sultone (see patent document 4 below), vinylene carbonate (see patent document 5 below), and vinyl ethylene carbonate (see patent document 6 below). Inter alia, vinylene carbonate has been widely used for its high effect. These additives are considered to form a stable film called a solid electrolyte interface covering the surface of a negative electrode, which film is expected to prevent reductive decomposition of an electrolyte.
The recent rise in the price of scarce metals, such as cobalt and nickel, has rapidly boosted use and development of a positive electrode material containing a low-cost metal material, such as manganese or iron. A manganese-containing, lithium transition metal oxide is one of positive electrode materials of current interest for its excellent performance, such as capacity and power, in lithium secondary batteries. It is known, however, that manganese is liable to be eluted from a manganese-containing lithium transition metal oxide as a positive electrode material in a lithium secondary battery to cause a side reaction, resulting in deterioration of battery performance, such as capacity and power.
To prevent elution of manganese from a positive electrode, various additives for a nonaqueous electrolyte have been proposed, including a disulfonic ester as disclosed, e.g., in patent document 7 below. There has still been a demand for further improvements.